1. Field of the Invention
This invention relates to a light source device for endoscopes which is capable of providing illumination light most suitable for rigid endoscopes, fiberscopes, and electronic scopes on color mosaic and field sequential systems.
2. Description of Related Art
In recent years, endoscopes have been widely used in which an elongated inserting section is inserted in a body cavity to thereby observe the internal organs. Various treatments are available which use a medical instrument inserted in a channel when necessary.
Endoscopes are roughly divided into two classes: rigid endoscopes or fiberscopes for visual observation, and electronic scopes using solid state image sensors, such as charge coupled devices (CCDs), as image pickups. As the image pickup systems of color pictures of the electronic scopes, available are a field sequential system for changing illumination light in sequence into R (red), G (green), and B (blue) light, which is disclosed, for example, in Japanese Patent Preliminary Publication No. Sho 61-82731, and a color mosaic system (or simultaneous system) having a filter array, in front of a solid state image sensor, in which filters transmitting R, B, and G light are arranged in a mosaic manner, which is disclosed, for example, in Japanese Patent Preliminary Publication No. Sho 60-76888.
In general, each of such rigid endoscopes, fiberscopes, or electronic scopes is connected to a light source device which is capable of providing illumination light suitable for each scope. However, rigid endoscopes, fiberscopes, or electronic scopes on the color mosaic system differ in illumination technique from electronic scopes on the field sequential system. That is, the former requires white light, whereas the latter requires light to be changed in sequence into R, G, and B light. Thus, it is necessary to provide a particular light source device according to the type of scope used. Moreover, operation varies with the light sources used. In this way, various problems have arisen as to economical and working efficiency.
To solve such problems, a light source device for endoscopes such as that disclosed in Japanese Patent Preliminary Publication No. Sho 63-281117 is known which is capable of providing illumination light, by itself, suitable for the electronic scope on the field sequential system as well as for the rigid endoscope, fiberscope, and electronic scope on the color mosaic system.
This device, however, emits light in the infrared region of the spectrum most copiously, in addition to light in the visible region which is required for observing an object, and thus has the problem of burning the fiber end of a light guide at which light from the light source concentrates.
In order to overcome this problem, for example, as set forth in Japanese Patent Preliminary Publication No. Sho 57-5020, provision has been made for mounting an infrared absorbing filter between the light source and the light guide to prevent light on the long wavelength side of the visible region from entering the light guide so that the fiber end of the light guide is not burned. Since, however, this technique uses only one infrared absorbing filter, the temperature of the base plate of the filter is raised by absorbed infrared rays when illumination is provided for a long time. Thus, the base plate of the filter itself cannot withstand a resultant thermal expansion and will cause breakage.
Infrared cutoff filters that are available include an infrared reflecting filter in which a coating consisting of a multilayered interference film applied to a transparent base plate and its coated surface possesses the properties of reflecting infrared rays and of transmitting the other rays, and an infrared absorbing filter composed of a material absorbing infrared rays. In general, the infrared absorbing filter has a spectral transmittance characteristic such as that shown in FIG. 1, and excels in the fact that most of the light belonging to the infrared region can be blocked. If, however, the infrared absorbing filter is used by itself, as mentioned above, the base plate of the filter cannot withstand thermal expansion produced by absorbed heat and may be broken. For this reason, for example, as set forth in Japanese Utility Model Preliminary Publication No. Hei 3-51411, a technique has been used that a reflecting filter is interposed between an illumination light source and an absorbing filter so that light emitted from the illumination light source is first incident on the reflecting filter, which reflects part of the light in the infrared region, and then the remainder is absorbed by the absorbing filter to block infrared light separately. According to the embodiment of this prior art publication, the spectral transmittance characteristics of the infrared reflecting filter and the infrared absorbing filter are as shown by curves A and B, respectively, in FIG. 2.
However, even with the light source device using the arrangement of the infrared cutoff filters mentioned above, illumination provided for a relatively long time causes the entrance end of the light guide to be burned, with a resulting extreme decrease in the amount of illumination light emerging from the exit end of the light guide. Furthermore, if the light source device is used for a long time, the absorbing filter will suffer deformation and breakage because of heating.
Where a xenon lamp is used as the illumination light source for example, the causes of burning occurring at the entrance end of the light guide and of breakage in the infrared absorbing filter can be explained as follows:
FIG. 3 shows the characteristic curve of the spectral energy emissivity of a common xenon lamp. According to this diagram, the xenon lamp emits, at a very high rate, light in the infrared region of the spectrum with wavelengths of 750-1100 nm. On the other hand, according to the spectral transmittance characteristic curves of the infrared cutoff filters of the conventional light source device shown in FIG. 2, the infrared reflecting filter disposed subsequent to the illumination light source transmits most of light of wavelengths 750-800 nm and light of wavelengths 800-1100 nm at a transmittance of at least 10%. After that, the amount of transmitted light increases with increasing wavelength. The infrared absorbing filter placed behind the infrared reflecting filter completely absorbs light of wavelengths more than 900 nm, but does not completely absorb light of wavelengths 700-900 nm and transmits part thereof. Consequently, of the light of wavelengths 750-1100 nm emerging from the illumination light source, light of wavelengths 750-900 nm is not completely removed by the infrared cutoff filters and concentrates at the entrance end of the light guide. In this way, the entrance end of the light guide is burned.
The infrared absorbing filter practically absorbs infrared light of wavelengths more than 700 nm which is not completely removed by the infrared reflecting filter, and radiates heat itself. Since, as is generally known, a heating element radiates infrared light itself, therefore for the infrared absorbing filter absorbing infrared light from the illumination light source generating heat becomes a new infrared radiation source (hereinafter referred to as "a secondary light source"). Part of the infrared light emitted from this secondary light source concentrates at the entrance end of the light guide and causes burning of the entrance end.
As the amount of infrared light absorbed by the infrared absorbing filter increases, the amount of generated heat increases and the amount of infrared light emitted from the secondary light source also increases. Hence, in order to prevent the entrance end of the light guide from being burned, it is required that the amount of infrared light absorbed by the infrared absorbing filter be controlled by an infrared reflecting filter disposed on the light source side of the infrared absorbing filter. In the conventional infrared reflecting filter, as shown in FIG. 2, infrared light of wavelengths 750-1100 nm, emitted at a high rate by the xenon lamp, is not sufficiently blocked. Thus, the infrared absorbing filter mainly absorbs infrared light in this wavelength range and radiates heat to form the secondary light source, which emits a sufficient amount of infrared light to burn the entrance end of the light guide. In this case, the surface temperature of the infrared absorbing filter is as high as 400-450.degree. C., and the infrared absorbing filter suffers from a strain because of thermal expansion and yields deformation and breakage after long term use.
Some adhesives for bonding the fiber bundle of the light guide absorb ultraviolet light. In a light guide using such an adhesive, ultraviolet light emitted from the illumination light source is responsible for the burning of the entrance end of the light guide. Since conventional infrared cutoff filters transmit most light with wavelengths shorter than 400 nm or less, light in this wavelength range is absorbed at the entrance end of the light guide to radiate heat. This situation facilitates the burning of the entrance end of the light guide.
According to the conventional infrared cutoff filters, as mentioned above, of light emitted from the illumination light source, light with wavelengths shorter than 400 nm and light with wavelengths longer than 750 nm are not completely blocked. Furthermore, the removal of infrared light of wavelengths 750-1100 nm is insufficient because the infrared reflecting filter is disposed closer to the illumination light source. In this way, there are problems of burning the entrance end of the light guide and of breaking the infrared absorbing filter.
The infrared reflecting interference filter cannot transmit 100% of the light in the visible region, resulting in decrease of the amount of light in the visible region by about 10%. On the other hand, the infrared absorbing filter absorbs not only infrared light, but also visible light, and decreases the amount of visible light by about 20%. Hence, if the infrared reflecting interference filter and the infrared absorbing filter are used in combination with each other, the amount of transmitted light in the visible region will be considerably decreased.
Where the electronic scope on the field sequential system is used, a color separating filter on the field sequential system is inserted in the optical path, in addition to the infrared absorbing filter. This color separating filter separates, in sequence, R, G, and B components from white light and transmits them, and thus the brightness of the transmitted light is reduced to about 1/3 compared with the case where the color separating filter is not inserted in the optical path. The problem is thus encountered that when the electronic scope on the field sequential system is employed, brightness, as illumination light, is insufficient.
While the infrared light emitted could burn the fiber end of the light guide and thus must be blocked, visible light also is responsible for a rise in temperature at the fiber end of the light guide because it has a considerably high energy density at a light collecting section, namely the fiber end of the light guide.
Therefore, where the rigid endoscope, fiberscope, or electronic scope on the color mosaic system is used, light in the visible region is concentrated simultaneously at the fiber end of the light guide, and the energy density of light in the visible region is increased, with the result that the temperature at the fiber end of the light guide is raised. In order to prevent strictly the burning of the fiber end of the light guide, it is necessary to remove light completely, at least, in the infrared region.
Where the electronic scope on the field sequential system is used, on the other hand, white light emitted from the light source is separated to produce, in sequence, R, G, and B components, which are concentrated at the fiber end of the light guide. In this way, the energy density of light in the visible region at the fiber end of the light guide is reduced to nearly 1/3 compared with the case where the rigid endoscope, fiberscope, or electronic scope on the color mosaic system is used. Thus, the temperature at the fiber end of the light guide is not very high, and even though light in the infrared region emitted from the light source is not completely eliminated, the fiber end of the light guide will not be burned in the case where the rigid endoscope, fiberscope, or electronic scope on the color mosaic system is used.
However, in the light source device capable of providing illumination light suitable for any and all of the conventional rigid endoscopes, fiberscopes, and electronic scopes on the color mosaic and field sequential systems at least two of the infrared reflecting interference filter and the infrared absorbing filter have been used to block almost completely light in the infrared region. When the electronic scope on the field sequential system is employed, light in the infrared region has been removed in excess of need.
Such excessive removal of light in the infrared region, as shown in FIG. 1, accompanies the removal of light in the visible region and reaches a considerable amount. This impairs brightness required for illumination light. Also, removing light in the entire infrared region by using interference filters alone to improve the brightness is expensive and therefore unprofitable.